1. Field of the Invention
The embodiments relate to optical spectroscopy. In particular, the embodiments relate to spectrometers. More particularly, the embodiments relate to compact spectrometers designed to reduce and minimize their dimensions and volumes with optimized spectral performance characteristics based on the unilateralized optical technique described herein.
2. Description of Related Art
Instruments used for spectroscopic measurements and applications belong to one family that includes monochromators and spectrometers. A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light chosen from a wider range of wavelengths available at the input. A spectrometer is an optical instrument for measuring and examining the spectral characteristics of the input light over some portion of the electromagnetic spectrum, where the measured variable is often the light intensity.
A monochromator may be differentiated from a spectrometer in at least two aspects: (1) a monochromator has an exit slit positioned at its spectral focal plane; while a spectrometer has no exit slit, but a linear detector array mounted at its spectral image plane; and (2) a monochromator has to be equipped with a scanning mechanism driving either a dispersive grating, or a focusing mirror, or the exit slit, in order to transmit the desired monochromatic light as the output through the exit slit; while a spectrometer has no moving parts and is capable of acquiring an instant full spectrum of the input light.
Nevertheless, the optical systems of such kinds of spectroscopic instruments, regardless of whether the instrument is classified as a monochromator or a spectrometer, are the same in working principle. Therefore, monochromators and spectrometers often are considered the same kind of instruments. Further, for the sake of simplicity throughout this disclosure, only a spectrometer will be referenced in this disclosure. A typical optical system of a spectrometer basically comprises an element(s) for collimating, an element(s) for dispersing and an element(s) for focusing to form spectral images. An entrance slit functions as the input interface, where an optional input optics exists. A detector converts optical signals to electronic signals. Such conventional optical technique makes a spectrometer cumbersome, i.e., complex in construction, large in body volume and heavy in weight. Further, there exist a few technical problems inherently associated with such spectroscopic instruments, particularly for a conventional spectrometer: astigmatism over the spectrum on the detector plane, and field curvature from the spectrum focused onto the detector plane, as reviewed by U.S. Pat. No. 5,880,834.
As a result, it has become a challenge to design and build a spectrometer to overcome the drawbacks and technical problems mentioned above, to which, substantial efforts have been directed and numerous improvements have been published for the purposes of simplifying optics, minimizing body volume, reducing weight, and eliminating optical aberrations, mainly astigmatism and field curvature. Among those areas of concerns, constructing compact spectrometers has generated manifold attention since the trend in modern spectrometer systems is toward a compact one and it has the potential to open up for wider applications in many industries, as shown in the following.
Representatives of the art can be categorized in accordance of their construction features associated with spectrometers: lens spectrometers, mirror spectrometers, spectrometers of simple construction, and compact spectrometers.
Representative of the art for lens spectrometers is U.S. Pat. No. 3,572,933 (1971) to Boostrom, which discloses a monochromator of classical configuration comprising a collimating lens, a transmission grating and a focusing lens to form spectra. U.S. Pat. No. 5,497,231 (1996) to Schmidt discloses another lens monochromator of scanning feature, which relies on a reflective planar grating. U.S. Pat. No. 6,122,051 (2000) to Ansley discloses another lens spectrometer of multi slits, which uses a prism as dispersion element. U.S. Pat. No. 7,180,590 (2007) to Bastue et al. discloses another lens spectrometer of transmission path, which is independent of temperature-induced wavelength drift.
Representative of the art for mirror spectrometers is U.S. Pat. No. 5,192,981 (1993) to Slutter et al., which discloses a monochromator of Czerny-Turner geometry comprising a collimating mirror, a reflective grating and a focusing mirror. This configuration is one of those typical of early prior art efforts and is a technique that is generally well known. The improvement of the disclosure comprises the use of a single toroidal collimating mirror in the system in combination with a spherical focusing mirror to minimized optical aberrations within final spectral images.
Another representative of the art for mirror spectrometers is U.S. Pat. No. 6,507,398 (2003) to Arai et al., which discloses a spectrometer of crossed Czerny-Turner geometry where the incident beam and the reflected beam from the diffraction grating cross. Cross Czerny-Turner configuration becomes one of preferred considerations for compact spectrometer designs.
Another representative of the art for mirror spectrometers is U.S. Pat. No. 4,310,244 (1982) to Perkins et al., which discloses a monochromator of Fastie-Ebert geometry comprising a big mirror for both collimating and focusing, plus a reflective planar grating. Fastie-Ebert configuration evolves from that of Czerny-Turner by combining the two mirrors into one. It becomes a preferred choice for a design of simple construction, as disclosed by U.S. Pat. No. 6,081,331 (2000) to Teichmann, which describes a spectrometer of Fastie-Ebert geometry formed in a cylinder body of glass. U.S. Pat. No. 7,239,386 (2007) to Chrisp et al. also discloses a design of imaging spectrometer of Fastie-Ebert configuration, which is improved by a glass-immersed mirror and a glass-immersed grating. This modification provides extra optical power to compensate optical aberrations.
Representative of the art for spectrometers of simple construction is U.S. Pat. No. 4,568,187 (1986) to Toshiaki et al., which discloses a spectrometer comprising a single concave grating. The concave grating is manufactured with curved grooves of varied spacing for optimum performance, and functions for both dispersing and imaging. It has become a known art that a concave grating sets the minimum number of optical elements needed in a spectrometer, leading to a simplest structure form.
Another representative of the art for spectrometers of simple construction is U.S. Pat. No. 5,182,609 (1993) to Kita et al., which discloses a spectrometer of Rowland configuration, comprising a single concave grating plus a second optical element introduced in the path for flattening spectral image formed at the focal plane.
Another representative of the art for spectrometers of simple construction is U.S. Pat. No. 5,233,405 (1993) to Wildnauer et al., which discloses a double-pass monochromator comprising a lens for both collimating and focusing, and a reflective planar grating for dispersing.
Another representative of the art for spectrometers of simple construction is U.S. Pat. No. 5,424,826 (1995) to Kinney, which discloses an optical micro-spectrometer system. This system consists of a group of micro-spectrometers, each of which comprises an input fiber, a lens for both collimating and focusing, and a reflective planar grating for dispersing.
Another representative of the art for spectrometers of simple construction is U.S. Pat. No. 5,812,262 (1998) to Ridyard et al., which discloses an apparatus of spectrometer type for UV radiation. Constructed by a single piece of waveguide carrier, it comprises a concave mirror and a reflective planar grating for focusing light from the entrance aperture means onto the radiation detector means. This configuration relies on a fixed order of the optical elements of focusing and then dispersing the light, which makes it difficult to compensate or avoid aberrations, in particular chromatic aberration.
Another representative of the art for spectrometers of simple construction is U.S. Pat. No. 6,597,452 (2003) to Jiang et al., which discloses a Littrow-type spectrometer, comprising a planar mirror, a concave mirror for both collimating and focusing, and a reflective planar grating, arranged within a compact configuration.
Representative of the art for compact spectrometers is U.S. Pat. No. 5,159,404 (1992) to Bittner, which discloses a compact spectrometer where the grating and the focusing mirror are combined on one side of a single transparent carrier, and the light entrance means and light detecting means are both placed on the other side of the spectrometer, making it possible to construct a compact spectrometer with a robust body.
Another representative of the art for compact spectrometers is U.S. Pat. No. 5,550,375 (1996) to Peters et al., which discloses a compact spectrometer designed as infrared spectrometric sensor. It comprises two parts: single-piece shaped base mirror plate manufactured as a microstructured body, having the concave cylindrical grating formed at one end, and the entrance port and detector slit at the other end, and a thin plate mirror as top cover. The integrated spectrometer has a thin layer (less than 1 mm) of reflective hollow cavity, which is filled with the gas to be monitored, through which infrared light propagate in divergence and convergence laterally, but guided vertically by the top and bottom mirror surfaces. This structure is only suitable for infrared peak absorption measurement of gas using a single cell detector of large area.
Another representative of the art for compact spectrometers is U.S. Pat. No. 6,606,156 (2003) to Ehbets, et al., which discloses a compact spectrometer comprising a concave grating, mounted on one side of the housing. The input port and the detector array are positioned opposite the concave grating, leaving a hollow cavity where the input optical beams propagate.
Another representative of the art for compact spectrometers is U.S. Pat. No. 7,081,955 (2006) to Teichmann et al., which discloses a compact spectrometer comprising two parts: the main body with grating and the focusing element being formed on the top of the housing, and the bottom substrate of detector array with light entrance means. The integrated spectrometer has a hollow cavity where the input optical beams propagate.
Another representative of the art for a compact spectrometer is U.S. Pat. No. 4,744,618 (1988) to Mahlein, which discloses a waveguide based device as multiplexer/demultiplexer, where light propagates based on total internal reflection through micro structures. Functioning like a compact spectrometer, it has a unilateral-type solid monolithic glass body of the Ebert-Fastie configuration, which makes it possible to build a compact device.
As stated above, conventional spectrometers are cumbersome and have large volumes, including those compact spectrometers of a single concave grating, which are either constructed from a single solid block of transparent material (e.g., glass), or integrated by mechanical mounting parts and housing. In contrast, waveguide based spectrometers typically allow for smaller volumes. The difference in volume between conventional spectrometers and waveguide based spectrometers may be attributed to the fact that the former is constructed with bulky optical elements and has a light propagation path that is three-dimensional, while the later (i.e., a waveguide based spectrometer) is constructed from a thin monolithic glass substrate in which a light propagation path exists in a thin layer (e.g., approximately 10 to 100 s micrometers) of glass media that are two-dimensional, or at least substantially unilateral. It seems that waveguide based technology may becomes a promising candidate for compact spectrometers.
However, from a practical perspective, the manufacturing process of waveguide products is expensive, and there are other technical issues associated with waveguide performance, including, but not limited to, high propagation loss, stray light caused by scattering at waveguide boundary, etc . . . Additionally, the coupling efficiency of waveguide devices is very susceptible to misalignment at input interfaces.
In general, existing spectrometers have not been an object of miniaturization as has been other technological machines and equipment because of the lack of technology in such field of endeavor. Thus, wider applications of spectrometers have not been possible for areas where miniaturization has become increasingly necessary or preferable. The embodiments of this disclosure overcome the above-identified disadvantages.